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What the human genome project tells us | Gene anatomy and switches

The human genome project was completed in 2003. It was a huge collaborative project that yielded quite a few surprises.

(If you need a refresher on genes and genetics before jumping into genomes, take a look at last week’s post on genes and genetics.)

What is a genome?

The NIH defines a genome as “an organism’s complete set of DNA, including all of its genes. Each genome contains all of the information needed to build and maintain that organism. In humans, a copy of the entire genome—more than 3 billion DNA base pairs—is contained in all cells that have a nucleus.”

Human Genome project

The human genome project was a huge collaborative project to determine the sequence of the human genome and identify the genes it contains.

Before it was completed, scientists guessed we’d have around 100,000 genes. When the first rough draft was completed in 2001, the number fell to 30,000. Nowadays, the number seems more like 20,500 human genes. The actual number still remains a mystery. What is the relevance of the number?

The phrase “more than a chicken, less than a grape” has been bandied about relating to the number of genes a human has! Indeed, grape vines have 30,434 genes (latest count) and chickens have 16,736 genes so with ~20,500 genes, us humans fall somewhere in between!

It’s tricky to get actual numbers as the genes aren’t all laid out in a single continuous stretch of genetic code. Instead, genes are found in protein-coding pieces, interspersed with stretches of DNA that don’t make proteins. All these components make pinpointing the bits that make proteins difficult. Take a look at the anatomy of a gene from the University of Utah genetics website:

Structure

What this gene anatomy shows us helps explain why humans have a relatively low number of genes. Our 20,500 human genes make up less than 2% of our genome’s nucleotides. Another chunk contains non-coding genes which don’t make proteins. And the bulk of the genome doesn’t code for any product at all. But it provides structure and organization to keep our genes working.

Splicing

By putting together different combinations of exons (the coding parts) in a gene, the cell can make different mRNAs from the same gene and thus different proteins.

If you are old enough to remember cassette tapes…think of when part of the tape got mangled and you had this tape splicing kit. You could splice together different parts of the tape. That’s just what the gene seems to do. Different exons are spliced together, and so one gene can make more than one protein. Check the diagram link above, and visually it’ll make sense.

Switches

Before proteins can be made, the switches have to be activated. These gene switches are what give cells their own identity. They explain why a heart cell is different from a skin cell yet both have the same DNA. The switches regulate what genes are turned on and off so each cell has its own combination of active and inactive genes.

Switches can work in combination, and a single switch is able to activate multiple genes. This all adds to the complexity and flexibility of the cell.

These genetic switches are vital in our embryonic stage. They play a predominant role in laying out our basic body plan and early functions. The image at the top of this blog post shows just this. On the left are heart cells and on the right are nerve cells. The DNA inside these cells is exactly the same. What makes the cells differ in form and function is that different genes are turned on or off in each type of cell – gene expression.

But we have more than just genetic switches, we also have epigenetic switches that can affect gene expression. In the next couple of weeks, we’ll look further into gene expression by exploring the epigenome, epigenetics, nutrigenomics and nutrigenetics. And we’ll figure out what we can learn from having our genes analyzed. Stay tuned.

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